javax.servlet.jsp.el
Class ExpressionEvaluator

The abstract base class for an expression-language evaluator.
Classes that implement an expression language expose their functionality
via this abstract class.

An instance of the ExpressionEvaluator can be obtained via the
JspContext / PageContext

The parseExpression() and evaluate() methods must be thread-safe.
That is, multiple threads may call these methods on the same
ExpressionEvaluator object simultaneously. Implementations should
synchronize access if they depend on transient state. Implementations
should not, however, assume that only one object of each
ExpressionEvaluator type will be instantiated; global caching should
therefore be static.

Only a single EL expression, starting with '${' and ending with
'}', can be parsed or evaluated at a time. EL expressions
cannot be mixed with static text. For example, attempting to
parse or evaluate "abc${1+1}def${1+1}ghi" or even
"${1+1}${1+1}" will cause an ELException to
be thrown.

parseExpression

Prepare an expression for later evaluation. This method should perform
syntactic validation of the expression; if in doing so it detects
errors, it should raise an ELParseException.

Parameters:

expression - The expression to be evaluated.

expectedType - The expected type of the result of the evaluation

fMapper - A FunctionMapper to resolve functions found in
the expression. It can be null, in which case no functions
are supported for this invocation. The ExpressionEvaluator
must not hold on to the FunctionMapper reference after
returning from parseExpression(). The
Expression object returned must invoke the same
functions regardless of whether the mappings in the
provided FunctionMapper instance change between
calling ExpressionEvaluator.parseExpression()
and Expression.evaluate().

evaluate

Evaluates an expression. This method may perform some syntactic
validation and, if so, it should raise an ELParseException error if
it encounters syntactic errors. EL evaluation errors should cause
an ELException to be raised.

Parameters:

expression - The expression to be evaluated.

expectedType - The expected type of the result of the evaluation

vResolver - A VariableResolver instance that can be used at
runtime to resolve the name of implicit objects into Objects.

fMapper - A FunctionMapper to resolve functions found in
the expression. It can be null, in which case no functions
are supported for this invocation.